6.4.4. Future Scenarios

Direct RF from sulfate and BC in the future is obtained by scaling the best
values for 1992 to future fuel use (see Table 6-1).
By 2050, it is projected to increase by factors of about 2-5 for F-type scenarios
and 7-10 for E-type scenarios. Atmospheric levels of aircraft sulfate and BC
are assumed to respond instantaneously to fuel burn. RF values for these future
scenarios given in Table 6-1. For the central FESGa
(tech1) 2050 scenario, RF(sulfate) is estimated to be -0.009 (-0.003 to -0.027)
W m-2, and RF(BC) is estimated to be +0.009 (+0.003 to
+0.027) W m-2. The magnitude of radiative forcing for
sulfate and BC aerosol from subsonic aircraft appears to cancel, but this appearance
is deceptive because EI(S) and EI(BC) are highly uncertain for the future fleet
and are not coupled. For the range of scenarios listed in Table
6-1, each of these RFs remains smaller than the RF from CO2,
O3, or persistent contrails; these effects still need
to be considered, however, especially in the upper limits of the uncertainty
range.

We do not evaluate here the climate impact of sulfate and BC aerosols from
the projected HSCT fleet (scenario Fa1H). Sulfate released near 20 km would
augment the natural Junge layer, adding about 25% to total mass and a smaller
fraction to reflectivity (see discussion in Chapter 4).
These numbers depend on the sulfur content of HSCT fuel. BC aerosols released
from HSCT aircraft would primarily heat the stratosphere and may lead to a small
negative value of RF after stratospheric adjustment. Still, the EI(S) and EI(BC)
from yet-to-be-developed HSCT aircraft are highly uncertain but are likely to
be much smaller than other HSCT-induced RF.

If RF from contrails of +0.02 W m-2 in 1992 (see Section
3.6) scales with fuel burn, it would increase to +0.06 W m-2
by the year 2050 (Fa1). However, contrails are expected to increase more rapidly
than global aviation fuel consumption as a consequence of a number a factors:
Air traffic is expected to increase mainly in the upper troposphere, where contrails
form preferentially; newer, more efficient engine/airframes will travel greater
distances with the same amount of fuel, but larger wide-body aircraft carry
more passengers and burn more fuel for the same distance; and more efficient
aircraft can trigger contrails at higher atmospheric temperatures, hence at
a larger range of altitudes (see discussion in Section 3.7
and Gierens et al., 1999). Thus, global mean RF for persistent contrails is
predicted to be larger by an additional factor of 1.6 (+0.10 W m-2).
Technology option 2 (scenario Fa2) does not increase contrail RF because the
same distances are flown although the fuel burned is greater.

The radiative forcing from aircraft-induced cirrus clouds in 2050 is even more
uncertain than for 1992 (Chapter 3). An estimate could
fall between 0 and 0.16 W m-2 for the 2050 FESGa (tech1) scenario.

Persistent contrails in the stratosphere are not likely because of the low
ambient relative humidity. Radiative forcing from contrails from the proposed
HSCT fleet may therefore be neglected in future scenarios of radiative forcing
(except for the 11% reduction of subsonic RF from air traffic displaced by HSCT
aircraft).

6.4.5. Uncertainties

It is difficult to constrain the direct and indirect climate effects of aerosols
from aircraft. Measurements at altitude are not adequate to define the aircraft
contribution today, so models representing emissions and subsequent chemical
and physical transformations have adopted differing parameterizations to estimate
atmospheric concentrations. Furthermore, different aerosol size distributions,
the hygroscopic nature of some of the aerosol constituents, and mixing of different
species of aerosol all lead to uncertainties in the radiative properties of
aerosols. These uncertainties in the burden and radiative properties of aerosols
emitted by aircraft lead to associated uncertainties in radiative forcing that
are much larger than for a well-mixed, directly emitted greenhouse gas such
as carbon dioxide.

RF from aircraft emissions of greenhouse gases and aerosols can be constrained
when the amount of radiatively important species is directly limited by emissions
(e.g., CO2, H2O, sulfate, BC).
However, it is more difficult to place upper limits on RF when the climate impact
occurs indirectly, as in aircraft NOx production of O3
or-specifically here-in perturbations to naturally occurring clouds by aircraft
aerosols.

The indirect effect of aerosols from aircraft emissions on naturally occurring
clouds cannot be quantified at present owing to the complexity of several processes
such as ice-cloud nucleation and the dependence of albedo and emissivity on
the size of the ice crystals. With our present knowledge, this uncertainty must
be considered significant. Radiative forcing from contrails is estimated to
grow disproportionately with fuel use, and extrapolation to large values in
2050 is an important uncertainty in the future radiative impact of aviation.
Further uncertainty arises from the fact that all estimates of contrail RF were
made for the present climate, hence do not account for future climate change
(see discussion in Section 3.7.1).